Particle>Hair>Collection>Position?
Go to the object(s) in the collection. Go to edit mode (for each one if you have more objects) select the bottom two vertices, then hit SHIFT + S and choose Cursor to selected. Then go out of edit mode, hit CTRL + SHIFT + ALT + C (yes all at once) and choose origin to 3D cursor. Now the objects should be positioned on top of the plane in your particle system.